The present invention relates to saving power in disk drive idle modes, and in particular to reducing the power required by the disk head motor.
A disk drive typically comprises one or more disks in a stack, a spindle motor to rotate the disks, and one or more heads (transducers) which are moved across the disc surface by a Voice Coil Motor (VCM). The heads literally float on a cushion of air over the disk, generated by the rotation of the disk. When the disk is turned off, the heads are withdrawn so they don't come to rest on the disk and damage data or the head when power is turned off. In some designs, the heads are moved to an inner or outer crash stop and held in place by a permanent magnet field that doesn't require current to maintain.
During a standby mode, when a disk drive is not being accessed, it is common to save power by putting the disk drive in an idle mode. Completely powering down the disk drive is not typically done because that would require too much time to move the head back over the data region of the disk when the next data request is received. Rather, the spindle motor is slowed down to conserve power, while still causing the disks to spin fast enough to generate an air cushion to keep the head off the disk.
To position the heads over the disk, the typical hard disk drive has a head stack assembly (HSA) that includes a transducer, a pivotally mounted actuator arm for supporting the head, a VCM for exerting a torque onto the actuator arm, and a servo-controller for controlling the VCM. The VCM is a coil of wire next to a permanent magnet. The servo-controller moves the actuator arm by sending a control current through the coil which causes the permanent magnet to apply a force to the coil, which exerts a torque on the actuator arm.
To maintain the head inside the data area of the disk during idle, some current to the VCM is required. Without such current, a flex bias acting on the head will cause it to drift to the outside of the disk. This flex bias is a combination of factors, including the actuator arm flex torque and the air flow caused by the spinning disk acting on the head. Conventionally, during the VCM idle time, using a linear current control loop (CCL) control, a current in the VCM creates the torque required to overcome the flex bias to maintain the head inside the media. The power dissipation that is required during the linear current control loop (CCL) operation is equal to VDD*(Motor current+Quiescent current).
Due to continued advances in the state of the art of low power technology, it has become more important to extend system battery life for mobile hard drives. It would be desirable to further reduce the power required during an idle mode.